Untie Symbols class from CommonBackendContext

`Symbols.context` property is actually unused. Let's drop
it as it makes harder to create an instance of this class.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/JsIrBackendContext.kt

@@ -199,7 +199,9 @@ class JsIrBackendContext(
         .let { symbolTable.referenceSimpleFunction(it!!) }
 
     override val ir = object : Ir<JsIrBackendContext>(this, irModuleFragment) {
-        override val symbols = object : Symbols<JsIrBackendContext>(this@JsIrBackendContext, irBuiltIns, symbolTable) {
+        override val symbols = object : Symbols(irBuiltIns, symbolTable) {
+            private val context = this@JsIrBackendContext
+
             override val throwNullPointerException =
                 symbolTable.referenceSimpleFunction(getFunctions(kotlinPackageFqn.child(Name.identifier("THROW_NPE"))).single())